The present application relates to a gel electrolyte and a gel electrolyte battery. More particularly, the present invention relates to a nonaqueous gel electrolyte obtained from a nonaqueous solvent gelled with a matrix polymer and a nonaqueous gel electrolyte battery using the same.
In recent years, as power sources for portable electronic devices, batteries are very important from an industrial point of view. For reducing the size and weight of electronic devices, batteries which are lightweight and efficiently use spaces in the devices are demanded. For meeting the demand, lithium batteries having large energy density and power density are the most promising.
Especially, batteries having high selectivity of shape, sheet-type batteries having a reduced thickness and a large area, or card-type batteries having a reduced thickness and a small area are desired, but the method employed in the past, in which a metal can is used in the external packaging of a battery makes it difficult to produce a battery having a reduced thickness and a large area.
For solving the problem, studies are made on batteries using a gel electrolyte obtained by adding a substance having a certain bonding action to a liquid electrolyte, or by gelling a liquid electrolyte with a polymer. In these batteries, the electrode and the electrolyte have an adhesive force therebetween, and hence the battery elements including the electrolytic solution can be fixed together. Therefore, no strong metal external packaging is needed, and an external packaging in a film form can be used, enabling production of a thin, lightweight, and inexpensive battery.
A nonaqueous electrolyte has poor ion conduction properties, as compared to an aqueous electrolytic solution used in an alkaline battery or a nickel-cadmium (Ni—Cd) battery. Water is a unique and excellent solvent having two features that the viscosity is so low that ions easily move in water and that the permittivity is so high that a salt is easily dissolved in water.
In the nonaqueous electrolyte, a mixed solvent of a solvent having a low viscosity and a solvent having a high permittivity is generally used. As the former, dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), or diethyl carbonate (DEC), ethyl butyl carbonate (EBC) which is a linear carbonate, is used, and, as the latter, ethylene carbonate (EC) or propylene carbonate (PC), which is a cyclic carbonate, is often used.
Examples of materials for the matrix polymer in a polymer battery include polyethers, such as polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyethylene oxide (PEO), and polypropylene oxide (PPO), and acrylate resins, such as polymethacrylate.
An acrylate resin is produced mainly by a crosslinking method. A battery having electrodes and a separator contained in external packaging and having no electrolytic solution is prepared, and an electrolytic solution, monomers, and a polymerization initiator are mixed together and charged into the battery. After charging the mixture into the battery, the mixture is gelled by crosslinking due to heat or the like. A production method similar to that for a conventional, liquid electrolyte rectangular battery can be employed. In addition, this battery has a great advantage in that electrolytic solutions having arbitrary compositions can be used.
However, the step of merely charging the electrolytic solution containing monomers into the battery makes it difficult to appropriately control the gel amount at an interface between the electrode and the separator. The electrolyte formed at an interface having a large distance between the electrode and the separator upon charging the electrolytic solution has an increased thickness, thus lowering the ion conduction properties. An unsatisfactory gel electrolyte is formed at an interface having a small distance between the electrode and the separator, and has only poor adhesion, so that lithium deposition is caused upon charging, thus lowering the cycle characteristics. There is a possibility that the polymer matrix is not completely impregnated with the electrolytic solution, forming a portion having no electrolyte. Such an uneven gel electrolyte causes uneven reactivity, leading to deformation of the battery or deterioration of the cycle characteristics.
Polyether has properties such that polyether itself can be used as a solid electrolyte and there is no need to form a gel electrolyte from polyether using an electrolytic solution, and therefore studies have been made on polyether. Polyether can also be used in the form of a gel with an electrolytic solution, and the gel can be produced by mixing the polymer with a molten electrolyte and cooling the resultant mixture, or polymerizing monomers. However, the negative charge of oxygen atoms in polyether attracts the positive charge of lithium cations to inhibit the lithium ions from moving, and hence the polyether used as a gel electrolyte is not excellent in ion conduction properties, and therefore the polyether gel electrolyte is rarely used in lithium battery products.
Polyvinylidene fluoride is a material having such excellent chemical and electrochemical stability in a battery that it is used as a bonding agent for active material in the production of electrode. Unlike polyether, polyvinylidene fluoride has no interaction with lithium cations and hence is excellent in ion conduction properties.
As an example of the method for producing a gel electrolyte using polyvinylidene fluoride and a battery using the gel electrolyte, the following method has been proposed in a related art.
Gel was first prepared from a polymer solution, and the solvent in the gel is extracted using a poor solvent to polyvinylidene fluoride to form a sponge-like porous material having about 1 μm to 5 μm pores. Then, an arbitrary electrolytic solution is added to the resultant porous material to prepare a gel electrolyte. Subsequently, the porous material is formed on a separator or electrode surface, and then they are together rolled to form a battery, followed by charging of an electrolytic solution into the battery, thus producing a gel electrolyte and a gel electrolyte battery.
This method has an advantage in that the solution containing polyvinylidene fluoride dissolved used in the preparation of a gel electrolyte is independent of the solution for battery and therefore the electrolytic solution for battery is irrelevant to the restriction on the solvent used in the preparation of gel. Further, this method has a feature such that a porous material having excellent liquid absorption is formed by extraction and an arbitrary electrolytic solution is charged.
However, this method has the following problems:
(1) The steps in the method are complicated and cumbersome, thereby increasing the production cost.
(2) The gel, which is once formed on an electrode, must be subjected to cleaning and extraction.
(3) The poor solvent used in the extraction is mainly a protic solvent, such as ethanol or water, which is not suitable for the lithium ion battery, and the electrode must be well dried after the cleaning.
(4) Further, steps for charging the electrolytic solution into the battery and sealing the battery are required.
(5) After formed on a separator, the gel electrode must be well cleaned and dried similarly, and it is difficult to form a porous polymer film on a thin and flexible polyolefin separator.
For solving the problems, for example, Japanese Patent Application Publication (KOKAI) 2000-243447 (Patent Document 1) and Japanese Patent Application Publication (KOKAI) 2001-167797 (Patent Document 2) suggest a method in which a gel electrolyte membrane is formed without using the above-mentioned extraction method, has been proposed. In this method, as described in Patent Documents 1 and 2, an electrolytic solution and polyvinylidene fluoride are mixed to form a gel electrolyte membrane on an electrode, and there is no need to form a porous material by extraction, and an excellent lithium ion polymer battery can be provided.
However, in the method for forming a gel electrolyte membrane described in Patent Documents 1 and 2, the gel components are dissolved in a diluent solvent, and the solvent is evaporated so as to make the other components be gelled, and therefore the electrolyte cannot contain a large amount of a solvent having a low boiling point and a low viscosity, thus making it difficult to obtain a battery exhibiting satisfactory properties in an environment at very low temperatures.